1. Field of the Invention
This invention relates to the field of devices used to hold and spin a wafer during the application of photoresist. More specifically, this invention relates to centering wafers on a vacuum chuck and positioning the chuck/motor subassembly on which the semiconductor wafer rests during the application of photoresist.
2. Prior Art
In the manufacture of semiconductor products, it is often desired to apply photoresist to a semiconductor wafer. Typically this is done by placing the wafer on a platform where vacuum holds the wafer in place. This is referred to as a vacuum chuck. The chuck is mounted on a shaft which turns at approximately 5-6000 rpm. A certain amount of photoresist is placed on the spinning wafer and centrifugal force causes the photoresist to spread evenly over the surface of the wafer. Should the wafer not be centered on the chuck properly, or the shaft is not positioned properly, the photoresist will be thicker on one area of the surface of the wafer than in other areas. This results in lower yields from that wafer since subsequent process steps are based on estimated thickness of the photoresist layer. Additionally, if the wafer is not centered, or the shaft is not positioned properly, excess vibration could eject the wafer from the chuck, further reducing yield.
In the prior art, wafers are centered on the chuck in the following manner. An aluminum wafer, of the same dimensions as the process wafers and having a hole in the center, is placed on the vacuum chuck. There is also a hole in the center of the chuck itself. Using eyesight, these holes are aligned, resulting in the wafer resting in the middle of the chuck. At that time, teflon guides are placed on either side of the wafer, tightened in place, and the aluminum wafer is removed. During processing, the teflon guides place wafers on the chuck. Each wafer is placed on the chuck in the same location as the initial aluminum wafer. In order to operate effectively, the shaft/motor subassembly itself has to be positioned and leveled. When the shaft assembly is not positioned and centered, an uneven distribution of the photoresist on the surface of the wafer results. In addition, there is vibration in the chuck and the wafer which could eject the wafer from the chuck and result in lower yield. The shaft/motor assembly must be positioned, centered and levelled each time the machine is disassembled. In the prior art, this was done by aligning the shaft by sight, reassembling the machine, checking the location of the shaft, disassembling the machine, placing the shaft in a new location, reassembling the machine, again checking the location of the shaft by sight, and continuing the process until the shaft appears to be positioned and centered. Depending upon the experience of the user, the machine had to be assembled and disassembled 3 to 6 times with shaft centering and positioning taking 40 minutes to one hour to complete.
Disadvantages of the prior art wafer centering process are several. A first problem arises when the hole in the center of the wafer and the hole in the center of the chuck are not of the same diameter. If the hole in the wafer is larger than the hole in the chuck, the user must attempt to center the chuck hole within the hole in the wafer. When the hole in the wafer is smaller than the hole in the chuck, the user is essentially guessing where the center of the chuck hole is when aligning the wafer. Second, when placing the teflon guides around the wafer, the user must hold the wafer with one hand, to prevent it from slipping off center, and work the guides with the other hand. Third, when tightening the guides, the wafer must still be held in place with one hand while the other tightens the guides.
The disadvantage of the prior art method of positioning the shaft/motor assembly is the obvious time waste in disassembling and reassembling the housing repeated times, and if the final position is not correct, the uneven distribution of photoresist.
The present invention seeks to solve these problems with a simple yet effective solution.